Conventionally, DVD (Digital Versatile Disc) has been developed as an optical disc having a dense record of information. The DVD can afford to record the data of 2.6 MB on one surface of an optical disc by illuminating a beam of light having a wavelength of 650 nm onto the disc through an optical system having a numerical aperture NA of 0.6. The DVD is capable of recording video signals of nearly one home on one surface thereof.
In the meanwhile, the home-use video tape recorder has a basic recording time of nearly two hours. In order to secure the handling equivalent to the video tape recorder on the optical disc and player or information reproducing apparatus, there is a need for the optical disc to record much more data. Meanwhile, in order to make feasible the process such as editing by making the most of a random access function and the like as the features of the optical disc, there is a necessity of recording video signals of nearly three hours.
Furthermore, there is a demand for a high-density reproducing exclusive optical disc in the marketplace. The transfer rate for the digital HDTV (digital high definition television) offered by BS (broadcasting satellite) digital television broadcast is expected 20 to 24 Mbps. The recording of digital HDTV video signals in an amount of one movie, e.g. about two hours and half or 150 minutes, requires 22.5 to 27 GB=(20 –25 Mbps)/8(bits)/1000×150(min.)×60(sec.). Basically, the reproducing capability of an information reproducing apparatus is determined by NA/λ by using an objective lens NA and a read-out beam wavelength λ. It is accordingly possible to improve the optical-disc recording/reproducing density by increasing the NA and decreasing the λ. The current DVD uses an optical disc system employs λ=650 nm and NA=0.6, and a light-transmissive layer with 0.6 mm thick between the most outer surface and the reflection recording surface of the information recording layer of the optical disc.
For example, Japanese Patent No. 2,704,107 discloses an optical disc having a track pitch of (0.72–0.8)×λ/NA/1.14 μm, a pit width or upper width of (0.3–0.45)×λ/NA/1.14 μm and a pit bottom width or lower width of (0.2–0.25)×λ/NA/1.14 μm provided that a reproducing beam wavelength of λ[μm] and an objective lens numerical aperture is NA. With such an optical disc, the crosstalk between the adjacent tracks is suppressed in amount by selecting a particular pit form within the above range of track pitch.
In this related art, when determining a track pitch and a pit width, as shown in FIG. 1, a pit train of a single frequency is assumed to estimate as a crosstalk a signal amplitude of a basic frequency component in a photodetector output signal obtained when the reproducing light spot scans a point deviated a constant amount from a track center.
In order to record data with further density, the particular pit form of the related-art disc if simply converted on the assumption, for example, of λ=405 nm and NA=0.85 provides a track pitch TP of 0.301 to 0.334 μm, a pit upper width Wm of 125 to 188 nm and a pit lower width Wi of 80 to 100 nm. If the optical disc as defined by such related art is reproduced, the relationship between the track pitch and the pit width deviates from an optimal value. This results in a problem that the crosstalk signal amplitude and main signal amplitude representative of superiority/inferiority in RF signal characteristic exceeds −9 dB that is not practically problematic, thus making impossible to secure a sufficient system margin.